Yes, but the thought (plan) of committing any sin is a sin in and of itself.
True. But on examination there's more to it that has severe cause and effect consequences.
The thoughts themselves are at first beyond our control, but less so with proper praxis. The thoughts can be immediately rejected or they can be entertained and played with. This can lead to placing ourselves in a time and place of opportunity. Then comes the act of sin itself. And further, the continuation of this process leading to an addictive state making it harder and harder to avoid.
Each of the stages brings more suffering, not less.
The Church has practical methods to help. Chief among them is examination of conscience and the Sacrament of Reconciliation.
Most important is awareness and remembrance: remembrance of God's love and remembrance of the pain of sin that can get forgotten in its glamour.
This is as contrasted with on earth, wherein many times the sin is satisfied.
Unless we confuse pleasure with happiness, sin is never satisfied, it results in a never-ending addictive desire for more, and greater and greater unhappiness.
Hence my theory is that leaving a sinner with only the thought of sin is a lesser suffering.
You have a reasonable point of view. I was just thinking of the times in my life when I had major habitual sin. While it broke down in the end, that was CAUSED by God to bring me to Him. Before God broke me, I was just fine with the sin. Here, God isn't going to be bringing anyone back. That was my train of thought anyway. :)